The invention relates to a device for detecting particles of a fluid, in particular for magnetically detecting the particles.
By immunofluorescence, marked particles, particularly also cell populations, can be analyzed and separated using a FACS system (FACS: Fluorescence Activated Cell Sorting/flow cytometry) (D. Huh et al., Physiol. Meas. 2005, 26, R73-R98). For this purpose, a cell population is pumped though a capillary tube where individual cells are consecutively checked for their fluorescent properties by an external optical system. Immediately after fluorescence detection, the cells are individually atomized into droplets in a nozzle and the droplets with marked cells are electrically charged. By deflection of the charged droplets in an electric field, the marked cells can be separated from a large cell population. FACS sorters here employ multiple markers and operate at speeds of ˜105 cells/min. This method constitutes the industry standard. The disadvantage of FACS lies in the high procurement and maintenance costs, and in the complexity of the system which requires trained personnel to operate it.
In addition to FACS separation, systems based on immunomagnetic principles are described in the literature, the emphasis here being mainly on the detection of marked cells and less on separation. Macroscopic dipole and quadrupole separators are used, as well as SQUIDS1 (Superconducting Quantum Interference Device). The disadvantages of these magnetic approaches are their impracticability (no miniaturization possible) and the high costs. Dipole and quadrupole systems, like FACS sorters, are complex and expensive instruments which, because of their size, exhibit only low marked cell recovery rates. SQUIDS must be cooled to below 100 K to be operational. In addition, the cooling necessitates a complex detector design which undermines the high sensitivity of SQUIDS.
In modern cell separation systems, MACS (Magnetic Activated Cell Sorting) is used. Here paramagnetic nanoparticles coated with monoclonal antibodies are mixed with a cell suspension. The antibodies bind to the specific antigen on the cell surface. If the cell suspension passes through a powerful magnetic field in a column, the cell/nanoparticle complexes remain in the column, while the free cells momentarily flow through (negative selection). If the column is removed from the magnetic field, the cell/nanoparticle complexes can be recovered (positive selection). Cells attaching to microbeads are viable and the bead/antibody complex can be removed from the cell surface. MACS is completely FACS-compatible. Following magnetic separation (100× accumulation of up to 109 cells in 15 min)2, flow cytometry analysis with fluorescence marking can take place. The disadvantage of this process is the need for double marking (magnetic and fluorescence) in order to perform cost-effective analysis/refined separation of the cells.